Research On Subsystem Partitioning And Restoration Strategy After Blackout Of Power Systems

With the continuous increase of modern society’s dependence on electricity supply, the consequences of the blackout are more and more serious. As one of the most important measures of the power system security defense, black-start restoration has important practical significance to shorten the restoration process, restore the power supply to the user as soon as possible and reduce the economic losses and social impact caused by the blackout.In large-scale power system, there may be more than one black-start power sources which consist of black-start units or the external links. When the blackout occurs, the whole system is partitioned into several subsystems which can be restored in parallel by these black-start power sources, then interconnect them at the appropriate time to realize restoration of the whole power system. The failure of any subsystem restoration due to some unpredictable factors will not affect the other subsystems’ restoration process, thus this black start scheme has not only a higher speed but also a higher reliability in system restoration.The dissertation mainly studies the subsystem partitioning and restoration of large-scale power system. The formulation and technical check of black-start scheme, the algorithm of subsystem partitioning in the black-start restoration and the optimization of system partitioning are explored in depth. Main contributions in the dissertation are described as following:(1) The black-start restoration framwork is proposed combined with the actual situation of Hebei Southern power grid, and the black-start scheme is made on the basis of testifying some technical problems that may occur in restoration process, such as generator self-excitation, the power frequency overvoltage, energizing transmission line overvoltage, and the stability problem caused by parallel running of hydraulic unit and steam unit. The scheme has been verified by field tests.Based on the field test scheme, the impacts to the small system in the early stage of restoration caused by the energizing of no-load large capacity transformer is studied, and the feasibility of using a black-start unit stepping up from zero to start more than one units at the same time is demonstrated. The results show that it may pose a serious threat to the safety operation of the small system in early restoration when energizing no-load large capacity transformers, so some preventive measures must be taken to avoid the harmful effects. Meanwhile, the scheme of starting several units at the same time is feasible and it is important to improve the recovery efficiency and shorten recovery time.(2) Complex network theory is applied to optimization of system partitioning in the black-start restoration. Two partitioning methods are respectively proposed based on spectral clustering and similarity of node voltages. In the partitioning method of spectral clustering, the Laplacian matrix of the grid is defined based on the adjacency matrix and each node of grid is showed in the space derived from the first and the second small nontrivial eigenvector of the Laplacian matrix. Then use the clustering algorithm to achieve the subsystem partitioning. In the method based on similarity of nodes voltage, the system is divided into several subsystems by power flow calculation in specific way and selecting the appropriate voltage threshold. The partitioning method based on complex network theory can realize the goal of subsystems partition which has closely internal structures within subsystems and weak links among subsystems and it has important application value for the black-start restoration.(3) After extensive investigation of various time limitations in the system restoration, a parallel restoration method considering time limitation is proposed. Based on the investigation of stream unit starting limitation and recovery time, substation recovery time limitation and transmission line recovery time, a subsystem restoration algorithm is proposed which can reach the global optimal target that starting power plants and substations as many as possible in shortest time. Using the method, subsystem partitioning scheme and the restoration sequence in subsystem can be got by one time. The IEEE118 bus system is employed to demonstrate the validity of the partitioning scheme.(4) The reliability theory is applied in subsystem restoration, based on which a system partitioning method considering the reliability of the recovery operation is proposed. Considering the uncertainty of the equipment when putting into operation, the reliability of the recovery path from the charged region to the node to be restored is calculated and reliability indexes are brought into the optimization process. It avoids the drawbacks of weak grid structure due to one-sided pursuit of reducing restoration time. The restoration scheme is more reliable and can restore the substations as many as possible in the shortest possible time. The algorithm can also get the subsystem partitioning scheme and the restoration sequence in subsystem by one time. The IEEE118 bus system is employed to demonstrate the validity of the partitioning scheme.